In a film deposition apparatus such as a PVD apparatus, a CVD apparatus and an ion plating apparatus, an exposure apparatus, an etching apparatus, or the like has been conventionally carried out adsorption of a workpiece to be processed such as a silicon wafer onto a surface of a plate-shaped member finished flat and smooth in order to fix the workpiece to be processed with precision. A sample holder utilizing an electrostatic adsorption force is employed as this adsorption means.
In such a sample holder, a sample holding surface (an adsorption surface) is provided in an upper face of the plate-shaped substrate composed of ceramics. In the sample holder, for example, an adsorption electrode is provided in an inside of the substrate and then a DC voltage is applied to the adsorption electrode so that an electrostatic adsorption force such as a Coulomb force caused by dielectric polarization and a Johnsen-Rahbek force caused by a minute leakage current is generated relative to the workpiece to be processed and thereby the workpiece to be processed can be adsorbed and fixed onto the sample holding surface.
The sample holder is used in a state where a supporting member is joined to a lower face thereof. For example, the supporting member includes a water passage provided in an inside of the supporting member and is constructed from a metallic material such as aluminum. Indium or an indium alloy is used for the joining. When such joining is to be performed, an underlayer of silver, copper, or the like having satisfactory wettability with indium is formed in advance in both of joining surfaces of the sample holder and the supporting member. Further, in some cases, an alloy of indium and tin, silver, or the like for reducing the melting point of indium is interposed between both of the joining surfaces of the sample holder and the supporting member.
Nevertheless, in a case where an underlayer having satisfactory wettability with indium is merely formed in both of the joining surfaces of the sample holder and the supporting member, the joining strength between the sample holder and the supporting member is difficult to be improved. Specifically, in a sample holder described in Japanese Unexamined Patent Publication JP-A 2004-50267 (referred to as Patent Literature 1, hereinafter), joining is performed by using an intermediate layer whose melting point is lower than the melting point of indium. Thus, there has been a possibility that when the sample holder goes into a high temperature, softening occurs in the intermediate layer so that the joining strength goes lower. Thus, there has been a possibility that the flatness and the parallelism of the sample holding surface vary in association with temperature cycles. As a result, the flatness and the parallelism of the sample holding surface have been difficult to be maintained.